The present invention relates to a radar for the detection of obstacles. It can be applied especially to automobile vehicles where it can be used to detect obstacles, more particularly obstacles constituted by other preceding vehicles.
Radars designed for large-scale consumer applications must be sold at the lowest possible prices while at the same time complying with the technical performance characteristics needed for reliability or safety. This is especially so with level measurement radars, road traffic management radars and more specifically automobile radars used for speed regulation or obstacle detection. A speed regulation radar for automobiles is designed especially to detect the distance and speed between a carrier vehicle and the vehicle preceding it, in order to enable the carrier vehicle to adjust its speed in relation to that of the preceding vehicle, for example to comply with safety criteria. A radar of this kind must especially be operational whatever the atmospheric conditions.
A known approach to measuring the distance from an obstacle to a radar is to use the phase rotation of the signal sent out by the radar during its to-and-fro path from the radar to the obstacle. The signal sent out e(t) is defined by the following relationship: EQU e(t)=A cos(2.pi.Ft) (1)
where:
F represents the frequency of the radar signal,
t represents the time;
A represents the amplitude of the radar signal.
The phase rotation .psi. or phase shift obtained on this signal after propagation over a distance 2D corresponding to the to-and-fro path between the radar and the obstacle is defined by the following relationship: EQU .psi.=2.pi.F.tau. (2)
where .tau. represents the time taken by the radar wave to travel through the to-and-fro distance 2D. Indeed: ##EQU1##
where c represents the velocity of light.
From the relationships (2) and (3), it follows that: ##EQU2##
The relationship (4) shows that a simple phase measurement enables access to the measurement of the distance D between the radar and the obstacle. However, in practice, the measurement is ambiguous once the phase shift .psi. is greater than 2.pi., if the receiver has two reception channels in quadrature, or greater than .pi. if it includes only one reception channel. This prohibits a direct measurement of distance by means of the phase in a radar use where the frequency is commonly higher than 1 GHz. One approach usually implemented consists then of the use of a differential phase measurement corresponding to radar signals emitted and received on two different frequencies F.sub.1 and F.sub.2.
A sequence of transmission at a first frequency F.sub.1 gives access to a first phase shift .psi..sub.1 : ##EQU3##
Similarly, a sequence of transmission at a second frequency F.sub.2 gives access to a second phase shift .psi..sub.1 : ##EQU4##
From the relationships (5) and (6), the measurement of the distance D is deduced: ##EQU5##
Assuming: EQU .psi..sub.2 -.psi..sub.1 =.DELTA..psi.
and EQU F.sub.1 -F.sub.2 =.DELTA.F
We get: ##EQU6##
For reasons of cost, the radar devices used in civilian applications and especially in automobile applications often rely on homodyne solutions from the viewpoint of the transmitter and the receiver and on monostatic solutions from the viewpoint of the antenna.
However, this type of device has a major limit related especially to the presence of different noises whose power is far greater than the power of the thermal noise, especially in the lower part of the spectrum of the video signal at output of the mixer. These noises are related to the amplitude noise of the oscillator and of the leakages that result therefrom by mismatching of the circulator-antenna assembly, to the inherent noise of the mixer, to the variations of the apparent standing wave ratio (SWR) of the antenna in the presence of mechanical vibrations and to the presence of rain clutter or again to the splashing of water on the antenna of the radar. In practice, the noises related to the microwave components are greater when the radar works in millimetric wave mode, which is especially the case with automobile radars working at 76 GHz. In practice, the amplitude noise of Gunn type oscillators or of the transistors used for the oscillator function and the noise of the diodes used for the mixer function substantially disturb the estimation of the phase of the received signal and even make this estimation impossible when the frequency of the demodulated reception signal is low.
In the particular case of a radar for the regulation of the speed of automobile vehicles, it is important that the quality of the phase measurement should be preserved when the radial speed of the target detected by the radar is close to zero. Since the detected target is actually the vehicle preceding the carrier vehicle, a radial speed close to zero corresponds to a regulation mode. The Doppler frequency of the radar echo is then close to zero and the estimation of the distance is done on the basis of a particularly noise-infested phase measurement.
The aim of the invention in particular is to overcome this drawback despite the technical limits laid down by the choice of the homodyne radar principle.